The invention relates to an apparatus for measuring the contact pressure which is exerted by a winding compression element on a winding in a power device, for example a power transformer.
The windings, particularly in power transformers, are composed of a combination of conductors and insulating materials. The windings are a complex structure, with a basically cylindrical geometry. After manufacturing or winding the transformer, the windings are provided such that they are enclosed by the iron core of the transformer.
Transformer windings must be configured such that they have sufficient mechanical strength and robustness in order to withstand short-circuits and other mechanical forces throughout the entire life of the transformer. One particularly important aspect is the compression of the windings, which produces good mechanical robustness.
The pressure which acts on the windings in the transformer due to the compression decreases over the course of time as a consequence of a number of influences. This then results in the mechanical strength often being inadequate in the event of short-circuits, with an increased probability of destruction of the transformer. No apparatuses installed in transformers are known which would allow detecting changes in the contact pressure.
By way of example, FIG. 6 shows a typical profile of the decreasing contact pressure, which was determined off-line, in each case after a short-circuit had occurred. FIG. 7 shows a profile which was recorded off-line at planned time intervals. Transformer windings are normally compressed during production. There is no capability to detect any change during operation. Recompression for correction purposes is feasible only during extensive repair work, for example when one of the windings needs to be replaced.
Even when configurations are used through the use of which the compression is readjusted to a certain extent by mechanical elastic elements during the operating period there is in principle no objective information about the actual state of the compression of the winding, and hence no information about the state of the transformer.
It is accordingly an object of the invention to provide an apparatus for measuring the contact pressure of a winding compression element in a power transformer.
With the foregoing and other objects in view there is provided, in accordance with the invention, in combination with a power device having a winding and a winding compression element exerting a contact pressure on the winding, an apparatus for measuring the contact pressure, including:
a sensor having a sensor element reacting to one of pressure and strain, the sensor being placed in the winding compression element; and
the sensor being configured to communicate via a radio link.
The apparatus allows the resultant contact pressure to be detected by measurement at any time, and hence the residual contact pressure, which still exists after a relatively long operating period, as well.
According to another feature of the invention, an electronic checking device is operatively connected to the sensor via the radio link.
According to yet another feature of the invention, at least one sensor antenna is electrically connected to the sensor.
According to a further feature of the invention, the power device is a power transformer configuration having a transformer tank with a wall; the winding is disposed in the transformer tank; a radio-frequency bushing extends through the wall of the transformer tank; a checking antenna is disposed in the transformer tank; and the electronic checking device is provided outside the transformer tank and is operatively connected to the checking antenna via the radio-frequency bushing.
According to another feature of the invention, the electronic checking device and the sensor are configured such that the electronic checking device transmits a radio-frequency signal, the sensor receives the radio-frequency signal and converts the radio-frequency signal into a mechanical surface acoustic wave, which propagates in the sensor and is reflected in the sensor, the sensor then converts the mechanical surface acoustic wave back to an electromagnetic wave and transmits the electromagnetic wave to the electronic checking device, and the electronic checking device evaluates the electromagnetic wave received from the sensor.
According to another feature of the invention, the sensor element is an SAW (Surface Acoustic Wave) sensor element.
According to yet another feature of the invention, the sensor includes an encapsulation, the sensor element is hermetically sealed in the encapsulation, and the encapsulation is configured as an elastic deformation body.
According to another feature of the invention, the sensor is placed in a spring element.
With the objects of the invention in view there is also provided, a power device, such as a power transformer, including:
a winding;
a winding compression element exerting a contact pressure on the winding;
a measuring device for measuring the contact pressure exerted on the winding, the measuring device including a sensor having a sensor element reacting to one of pressure and strain, the sensor being disposed in the winding compression element; and
the sensor being configured to communicate via a radio link.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in an apparatus for the measurement of the contact pressure of a winding compression element in a power transformer, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.